1. Field of the Invention
The present invention relates to a multi-axis parellel kinematic machine for multi-axis positioning or machining, and more particularly to a multi-axis parellel kinematic machine that can be controlled easier and more precisely in the Cartesian coordinate workspace with controllable orientation.
2. Description of Related Art
With reference to FIG. 10, a conventional parallel kinematic machine comprises an extendable arm (50) and three control struts (52). One end of the extendable arm (50) is pivoted by a universal joint (56) that is connected to a stand (51). The opposite end of the extendable arm (50) is fixed to a mobile head (57). A gripping device or a tool head (58) is attached to the mobile head (57). Each control strut (52) is a cylinder with a piston. One end of each control strut (52) is pivotally connected to the stand (51) by a joint (53) such as a ball joint. The other end of each control strut (52) is pivotally connected to the mobile head (57) by another joint (54). When the control struts (52) are retracted or extended, the control struts (52) will rotate relative to the stand (51). The arm (50) will be pushed or pulled by the actuation of the three control struts (52), and the arm (50) will move in rotary relative to the universal joint (56). The gripping device or the tool head (58) can be moved to a desired position by means of the actuation of the control struts (52). Accordingly, the position of the gripping device or the tool head can be controlled.
However, because the position of the arm (50) is confined through the rotations of the joints (56 and 54), the gripping device or tool head (58) cannot be moved in pure linear displacements relative to the stand (51). To reach any point located in the three dimensional workspace, the angles of the gripping device or driving tool relative to the stand (51) are also changed. The conventional machine is usually not used as a three-axis machine with a Cartesian workspace of three linear axes. The conventional machine is more often required the co-working by another two-axis rotary table or rotary head and then becomes a five-axis machine. Control of such a five axis machine requires much expansive and sophisticated software and hardware.
The metrology system to measure and control the position of the tool head (58) is arranged on the arm (50). The Cartesian workspace position of the tool head (58) is by transforming the measurements of the extension and rotations of the arm (50). Due to the Hertz contact deformation, the motion of the universal joint (56) is not precise. The deformation error and sensor resolution error of the universal joint (56) is amplified by the large length of the arm (50). The position accuracy of the gripping device or tool head (58) is limited due to the motion inaccuracy of the universal joint (56) and the length error of the arm (50).
With reference to FIG. 11, another conventional parallel kinematic machine in accordance with the prior art comprises a base platform (60), a mobile platform (62), a parallelogram guiding mechanism (63), and multiple legs (66). The parallelogram guiding mechanism (63) consists of three upper rods (631), three lower rods (632), and an intermediate platform (633). The three upper rods (631) are in-parallelogram pivoted to the base platform (60) and the intermediate platform (633). The three lower rods (632) are also in-parallelogram pivoted to the mobile platform (62) and the intermediate platform (633). Because of the parallelogram of the three upper rods (631), the intermediate platform (633) is only allowed to move in pure linear displacements relative to the base platform (60). Also, because of the parallelogram connection of the three lower rods (632), the mobile platform (62) is only allowed to move in pure linear displacements relative to the intermediate platform (633). Consequently, the mobile platform (62) is moved in pure linear displacements relative to the base platform (60).
Each leg (66) is telescopically and pivotally connected between the base platform (60) and the mobile platform (62). A gripping device or a tool head (not shown) is attached to the mobile platform (62). The mobile platform (62) is actuated by the three telescoping legs (66). Because the mobile platform (62) is only allowed to move in pure linear displacements relative to the base platform (60), this conventional machine can be used in the three-axis applications with a Cartesian workspace. However, because there are multiple high compliant pivot joints involved in the guiding mechanism (63), the rigidity and motion precision of the parallelogram guiding mechanism (63) is not good. The metrology system to measure and control the position of the mobile platform (62) is arranged on the telescoping legs (66). The Cartesian displacements of the mobile platform (62) are estimated by transforming the measurements of the extension of the leg (66). The Cartesian position accuracy of the mobile platform (62) is limited due to the joint locations errors and initial length errors of the leg (66).
To overcome the shortcomings, the present invention tends to provide an improved parallel kinematic machine to mitigate or obviate the aforementioned problems.
The main objective of the invention is to provide an improved multi-axis parallel kinematic machine that can be controlled more easily and accurately. The parallel kinematic machine has a motion platform, a base platform, multiple driving struts, and a multi-axis Cartesian guiding mechanism. The multi-axis Cartesian guiding mechanism is purposed for precise guiding the motions of the moving platform in a Cartesian workspace with defined or controllable platform orientation. For a three-axis machine, the motion platform can be moved in a Cartesian workspace with fixed orientation relative to the base platform. For a five-axis machine, the motion platform can be moved in a Cartesian workspace with a controllable platform inclination relative to the base frame. More important, by fully utilizing the elastic averaging design principle, the multi-axis Cartesian guiding mechanism system is also a highly rigid and precise metrology frame that the displacements of the motion platform can be measured and controlled accurately.
Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.